In a variety of optical applications there is a need to convert light of one state of polarisation (SOP) into another. For instance, in a coherent light optical transmission system there is typically a requirement for at least one polarisation state controller to establish a match at the receiver between the state of polarisation of the information carrying signal and that of the local oscillator signal. Polarisation control by means of one or more polarisation state controllers may also form part of a transmission system that has a transmission path that includes optically in a series a number of optical amplifiers, the controllers being employed to take care of polarisation dependent effects liable to be exhibited by such amplifiers.
In optical communications applications the light, whose SOP is to be converted, is typically in zero order mode which, in an integrated optics environment, is resolvable into TE.sub.0 and TM.sub.0 mode components. In a zero order mode integrated optics environment therefore a polarisation state converter receiving for instance a TE.sub.0 mode input signal, delivers from its output a signal having TE.sub.0 and TM.sub.0 components.
An example of such an integrated optics polarisation state converter is described by P Albrecht et al in a paper entitled, "Integrated Optical Polarisation Splitter", IEE Colloquium Polarisation Effects, Oct. 22, 1990. That converter relies upon a direct conversion between TE.sub.0 and TM.sub.0 modes using an electrostatic field aligned at right angles to the direction of propagation of light in a waveguide. Conversion is relatively weak in this instance, and proceeds cumulatively only whilst the TE.sub.0 and TM.sub.0 modes remain in mutual phase coherence. Because phase coherence is difficult to achieve over the necessary lengths, specific and quite elaborate means have had to be provided for compensating phase mismatch on individual devices.